Stirrer

ABSTRACT

A device for casting of metal. The device includes a mold, a casting tube via which molten metal is supplied to molten metal in the mold in a region at a distance below a meniscus of the molten metal, and at least one stirrer including an iron core and a coil arranged around the iron core. The iron core is arranged to be elongated along the broad side of the mold and adapted to apply a magnetic field to the molten metal for stirring the melted metal. An upper part of the iron core is positioned at a distance from the meniscus of from 50 mm above the surface of the meniscus to 195 mm below the surface. The length of the iron core in relation to the length of the broad side of the mold is between at least 50% and 80% of the length of the broad side.

TECHNICAL FIELD

The present invention relates to a device for continuous or semicontinuous casting of metals, comprising a stirrer according to the preamble to claim 1.

BACKGROUND OF THE INVENTION AND PRIOR ART

During continuous or semicontinuous casting, a molten metal is supplied to a casting mould, hereinafter designated mould, in which it is cooled and formed into an elongated strand. Depending on its cross-sectional dimensions, the strand is designated BILLET, BLOOM or SLAB. During the casting, a primary flow of hot, molten metal is supplied to the cooled mould, in which the metal is cooled and at least partially solidifies into an elongated strand. The cooled and partially solidified strand then continuously leaves the mould. At the point where the strand leaves the mould, it has at least a mechanically self-supporting, solidified casing that surrounds a non-solidified centre. The cooled mould is open at two opposite ends in the casting direction and preferably connected to means for supporting the mould and means for supplying coolant to the mould and supporting means. The mould is preferably made of a copper-based alloy with good thermal conductivity.

From a casting box, also designated tundish, the molten metal is supplied to the mould via a casting tube that extends down in to the mould. The casting tube preferably extends so far down into the mould that it projects into the molten metal that exists therein. When the melt from the tube flows into the melt that already exists in the mould, it generates a so-called primary flow and a so-called secondary flow. The primary flow leads downwards in the casting direction, whereas the secondary flow leads from the region of the walls of the mould upwards towards the surface of the metal bath located therein, designated the meniscus, and downwards. In different metal bath, designated the meniscus, and then downwards again. The meniscus is covered by a layer consisting of casting powder intended to act as protection against the surrounding atmosphere and to minimize heat losses.

In different parts of the metal bath that is present in the mould, periodic velocity fluctuations occur during the casting process. Thus, upper and lower loops arise, in which the melt flows around in a manner known per se. Due to resonance phenomena, which are associated with the periodic oscillations of such loops, large bubbles, for example argon gas bubbles, oxide inclusions from the casting tube, and slag of various kinds from the surface of the metal bath will be transported far down in the casting direction, that is, far down into the cast strand that is initially formed in the mould. This results in inclusions and irregularities in the finished, solidified cast strand.

If the hot metal flow is allowed to enter into the mould in an uncontrolled manner, the flow will penetrate deep into the cast strand, which probably will have a negative influence on the quality and productivity. An uncontrolled hot metal flow in the cast strand may result in encapsulation of non-metallic particles and/or gas occlusions in the solidified strand, or cause casting defects in the inner structure of the cast strand. A deep penetration of hot metal flow may also cause a partial remelting of the solidified surface structure so that the melt penetrates the surface layer below the mould, which causes severe disturbances in production and a long downtime for repair.

Velocity variations caused by oscillating flow in the mould give rise to pressure variations at the meniscus, and, in addition, variations in height arise at the meniscus. When the velocity of flow and hence the turbulence at the meniscus become too high, this leads to slag being drawn down from the casting powder and further down into the solidified strand, and results in an increased risk of cracking due to uneven shell growth.

On the other hand, when the velocity becomes too low at the meniscus, there is a risk of temperature differences arising, which may lead to local solidification at the meniscus with ensuing risks of cracking and of slag particles adhering under the shell that is solidifying at the meniscus. It is thus important, especially at low casting speeds, to maintain a flow at the meniscus that is optimal with respect to the speed in order to supply heat for melting of casting powder while at the same time endeavouring to obtain a low turbulence. In the region around the casting tube, there is a considerable risk of a local, unfavourable flow or stagnation of the melt arising, which leads to cracks being formed in the cast strand. Further, the oscillating flow provides an unsymmetrical velocity downwards in the mould. In certain situations, the velocity on one narrow side of the mould may become considerably higher than on the opposite narrow side, which results in a heavy downward transport of inclusions and gas bubbles with an ensuing deterioration of the quality of the cast object.

From Japanese patent publication JP-57017355, it is known to arrange electromagnetic stirrers, wherein the stirrer in the vertical direction is placed such that the distance from its upper edge to the meniscus is larger than or equal to 200 mm at the long side of the mould for the purpose of preventing casting powder from being drawn down from the meniscus of the melt and further down into the cast strand. The dimension of the stirrer in relation to the broad side of the mould amounts to 0.4-0.7 times the dimension of the broad side of the mould (0.4-0.7)*b. This solution, however, is only intended to create stirring a certain distance down in the melt and does not completely solve the previously mentioned problems relating to velocity variations.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a device for continuous or semicontinuous casting of metals, especially intended for casting of slabs, which contributes to reduce or eliminate the disadvantages mentioned above. In particular, a device is aimed at which creates an even flow at the meniscus for different speeds of the inflowing melt.

This object is achieved by means of the device described in the introductory part of the specification, which is characterized in that the iron core is arranged such that its upper part is positioned at a distance from the meniscus that lies from 50 mm above the surface of the meniscus to 195 mm below said surface.

By this device, the metal flow at the meniscus is directed away from the narrow sides of the mould inwards towards the casting tube and uniformly across the whole width of the melt, and, in addition, a homogeneous flow configuration is obtained at the meniscus which provides the lowest turbulence when the flow is uniform across the whole mould width. With a stirrer placed as previously described, a sufficiently large counter-directed meniscus flow is obtained uniformly over the whole width of the casting mould while at the same time the turbulence is restricted. The location of the stirrer also contributes to obtain a good rotation of the melt around the casting tube and the installation of the stirrer is considerably simpler compared with prior art solutions. By arranging the stirrer as described above, the secondary flow is utilized in an optimum way while at the same time, with the help of the stirrer, it is modified so as to obtain a good symmetrical flow of the melt in the mould including a good horizontal flow of the melt around the casting tube, which promotes an even shell growth while at the same time the amount of inclusions in the finished strand is reduced. By an optimum flow is meant that the velocity of the melt at the meniscus (the secondary flow) is maintained at a constant level without varying in time while at the same time the velocity of the metal flow (the primary flow) directed downwards from the casting tube is to be kept at as low a level as possible to minimize the risk of inclusions accompanying the melt far down into the solidified strand. The dimension of the iron cores of the stirrer in the vertical direction is usually 240-280 mm.

According to an alternative embodiment, the iron core is arranged such that its upper part is positioned at a distance away from the meniscus that lies from 50 mm above the surface of the meniscus to 150 mm below said surface.

According to an alternative embodiment, the iron core is arranged such that its upper part is positioned at a distance away from the meniscus that lies from 50 mm above the surface of the meniscus to 100 mm below said surface.

According to a preferred embodiment of the invention, two stirrers are arranged symmetrically around the centre line of the broad sides of the mould and on both sides of said broad sides. Since the iron cores of the stirrers only need to cover part of the width of the cast strand, such a device provides a cost-effective solution since a good rotation of the melt around the casting tube as well as an even velocity profile over the thickness of the width of the cast strand are obtained.

According to a further embodiment of the invention, two stirrers are placed asymmetrically, on respective sides of the long sides of the mould. This embodiment provides advantages such as lower weight, lower power consumption and reduced influence of magnetic fields on the surroundings. In addition, the pole pitch is large, which results in a maximally effective stirrer.

Additional advantages and advantageous features of the invention will become clear from the following description and the other dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be explained in greater detail by means of various embodiments and with reference to the accompanying drawings.

FIG. 1 is an explanatory sketch of the device according to the invention.

FIG. 2 is a top view according to one embodiment of the device according to the invention.

FIG. 3 is an exploded view of a continuous casting device according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention will now be described by means of various embodiments.

FIG. 1 shows an explanatory sketch of the invention, comprising a mould 1 enclosing a melt 2 which is supplied to the mould 1 by means of a casting tube 3 lowered into the melt. The melt 2 is cooled and a partially solidified strand is formed. The strand is then moved continuously out of the mould 1. According to the invention, at least one stirrer 4 is arranged which has an iron core and a coil applied around it and, with the iron cores arranged so as not to cover the whole length of the broad sides of the mould but instead at least 50% of the broad sides of the mould and at most 80% of the broad sides of the mould, symmetrically about the centre line 5 of the mould 1 on both sides of the broad sides of the mould. The iron cores are arranged such that their upper parts are positioned at a distance from the meniscus that lies from 50 mm above the surface 7 of the meniscus to 195 mm below said surface 7, in order to create a rotating stirring of the melt below the meniscus 7 by means of a period low-frequency travelling field. By arranging the stirrers 4 as described above, a good rotating stirring of the melt in the mould, including a good stirring of the melt around the casting tube 3, are obtained. Furthermore, the fact that the stirrers 4 do not cover the whole mould width means that the normal flow pattern that arises, when the melt is supplied to the mould via the casting tube 3, is not adversely affected.

FIG. 2 shows an alternative embodiment of the invention, wherein the stirrers 8 are located asymmetrically on respective sides of the broad sides 10 of the mould 9 and arranged such that the upper parts of the iron cores are positioned at a distance from the meniscus that lies from 50 mm above the surface of the meniscus to 195 mm below said surface.

The invention is not limited to the embodiments shown, but may be varied and modified within the scope of the following claims. 

1. A device for continuous or semicontinuous casting of metal, comprising: a mold comprising two broad sides and two narrow sides, wherein a ratio of the broad sides to the narrow sides amounts to 2:1, through which a molten metal passes during a casting process, a casting tube via which a molten metal is supplied to a molten metal already present in the mold in a region at a distance below a meniscus of the molten metal, at least one stirrer comprising an iron core and a coil applied around, the iron core, wherein the iron core is arranged to be elongated along a broad side of the mold and a length of the iron core in relation to a length of the broad side of the mold is between 50% and 80% of the length of the broad side and is adapted to apply a magnetic field to the molten metal to stir said melt, and wherein the iron core is arranged such that an upper part of the iron core is positioned at a distance from the meniscus of from 50 mm above a surface of the meniscus to 195 mm below said surface of the meniscus.
 2. The device according to claim 1, wherein the iron core is arranged such that the upper part is positioned at a distance from the meniscus of from 50 mm above the surface of the meniscus to 150 mm below said surface of the meniscus.
 3. The device according to claim 1, wherein the iron core is arranged such that the upper part is positioned at a distance from the meniscus of from 50 mm above the surface of the meniscus to 100 mm below said surface of the meniscus.
 4. The device according to claim 1, wherein two stirrers are located symmetrically around a center line of the broad sides of the mold and on both sides of said broad sides.
 5. The device according claim 1, wherein two stirrers are located asymmetrically, on respective sides of the broad sides of the mold. 